Minimal reference space definitions are presented for complete active
space self-consistent field (CASSCF) and restricted active space
self-consistent field (RASSCF) descriptions of bond- breaking
reactions and compared to CASSCF wavefunctions using full-valence and
one-to- one active spaces. Potential energy curves for reactions
breaking both single and multiple bonds are computed using
multi-reference perturbation theory and multi-reference config-
uration interaction based upon these reference wavefunctions, and
results are compared to full configuration interaction benchmark
curves. This allows one to ascertain the ability of minimal reference
spaces to recover the strong non-dynamical correlation effects in
these systems by comparison to results with larger reference spaces
and to the exact results for a given basis set. Minimal reference
spaces, at a fraction of the cost, typically perform on par with much
larger reference spaces in computations which also include dynamical
electron correlation.